Large scale computer systems typically have a number of component parts such as processors, storage devices, and input/output units, and data is regularly transferred among them during the execution of system tasks. In distributed processing systems which generally employ several cooperating processors to perform system tasks, the problem of communications among the various units of the system can become very complex.
As distributed processing systems become widely used, attention is being given to standardizing the data communication systems which provide communications between the various parts that make up the distributed processing system. One type of communication system which has gained wide acceptance is the carrier sense multiple access--carrier detect (CSMA--CD) type of system, exemplified by the Ethernet.RTM., a multipoint access data communication system with collision detection, which is substantially described in Metcalfe et al., U.S. Pat. No. 4,063,220.
The data communication system described by Metcalfe et al. comprises a bit-serial receiver-transmitter network which provides a communication path for a plurality of using devices, such as processors, storage devices, and input/output units, that are connected to it. The network is centered around a communication medium, such as a coaxial cable, to which a plurality of transceivers are tied for communicating with each other across the medium. A plurality of corresponding using devices are connected to the plurality of transceivers by means of associated network interface stages. Each transceiver and interface stage combination constantly senses the data stream passing through the network, the interface stage constantly looking for data packets addressed to it, that is, to its associated using unit.
In addition to providing normal transmitting and receiving functions, the transceiver provides further control functions, utilized for aborting a transmission of data packets from the interface stage, when it senses a collision on the medium--herein referred to as a remote collision--of data transmitted by the associated and another interface stage. Also, the associated interface stage blocks its transmission of data when it senses the presence of other data on the medium.
While being advantageous from the standpoint of simplicity and versatility, conventional Ethernet--type data communication systems suffer from certain disadvantages that limit their application and usefulness. In such systems, each using device is connected to the communication medium at a separate tap via a dedicated transceiver and interface stage combination. Because of the deterioration of the medium's electrical characteristics caused by taps on the medium, the number of taps, and hence the number of using devices that can be connected to the medium, has been limited. Also, the use of a separate dedicated transceiver with each using device has made addition of using devices to the system costly. But, inter alia because of the way in which collision detection is accomplished in such systems--for example through a bit-by-bit comparison of signals transmitted and signals simultaneously detected by a transceiver on the medium--it has been thought that the use of a single transceiver to service more than one using device would radically alter the communication protocol as seen by the using devices.
There is another major limiting factor on the number of using units that can be connected to such a network: the loss of network throughput due to conflicting requests for the network communication medium. An increasing number of using units connected to the network increase the contention for the medium and make collisions between transmissions attempted on the medium increasingly common. Because collisions result in aborted transmissions, the increasing occurrence of collisions increasingly degrades the throughput of the network and results in an increasing loss of operating speed of the using devices. After a limited number of using units has been connected to a given network, its performance on the average is degraded to a point where it cannot practically support any further using units.